When will the bomb hit the submarine? If an aeroplane flying 1200m above water, observes a submarine 1800m below it.
This a simple question we just need to understand that the submarine appears closer to the pilot than it actually is.
!!"Real depth"/"Apparent depth"=mu!!
In our case, the submarine appears to be at !!600m!! below water level.
So the bomb has to travel a total of !!2000m!!.
Time taken !!sqrt((2h)/g)=sqrt(400)=20m/s!!